The present invention relates to a current limiter in an ignition apparatus for an internal combustion engine which serves to limit a primary current flowing in a primary winding of an ignition coil, thereby limiting a secondary current flowing in a secondary winding thereof. More particularly, it relates to such a current limiter which is able to improve stability in the operation of an ignition apparatus.
In general, internal combustion engines such as automotive gasoline engines have a plurality of cylinders for which the order of fuel injection, the order of ignition and the like are controlled in an optimal manner by means of a computerized electronic control unit or "ECU".
The ignition timing of the cylinders of such an engine is determined by cutting off the current supply to the primary winding of an ignition coil, and the secondary winding voltage developed across the secondary winding of the ignition coil upon cutting-off of the primary current supply is required to have a high enough energy to generate a spark between the electrodes of a spark plug which is connected to the secondary winding of the ignition coil. In addition, it is necessary to limit the secondary winding voltage thus generated to a suitable energy level which does not cause dielectric breakdown of electronic or electric components of the ignition apparatus, the breakdown voltages for the components being determined in accordance with rated resistant voltages predetermined for the components. To this end, a maximum value of the primary winding current has to be limited to a prescribed value. However, the magnitude of voltage, which is supplied from a DC power supply such as a storage battery to the ignition coil for proper ignition, varies depending upon the operating condition of the engine, so it is general practice for the ignition apparatus to be equipped with a current limiter for limiting the primary winding current to an appropriate level in accordance with the operating condition of the engine.
Current limiters as conventionally used are operated by the base-emitter voltage of a power transistor which drives an ignition coil for turning on and off the current supply to the primary winding thereof.
FIG. 3 illustrates the circuit arrangement of a typical example of such a type of current limiter generally used in an ignition apparatus for an internal combustion engine. In this figure, a DC power source 1 in the form of a storage battery, which generates a source voltage V.sub.B, is connected to an ignition coil 2 which has a primary winding 2a and a secondary winding 2b of which the latter is connected to one of electrodes of a spark plug 3, whose the other electrode is connected to ground. A power transistor 4 comprising a pair of transistors coupled to form a Darlington circuit has a common collector connected to the primary winding 2a of the ignition coil 2, and a base connected to a node between a resistor 5, which is connected to a node between the storage battery 1 and the primary winding 2a, and a collector of a drive transistor 6 which has an emitter connected to ground. The drive transistor 6 is incorporated in an ECU (not shown). A current limiter, generally designated by reference numeral 10, is connected between the emitter and the collector of the power transistor 4. The current limiter 10 includes a current sensing resistor 11 connected between the emitter of the power transistor 4 and ground for sensing a primary voltage V.sub.D corresponding to a primary current I.sub.1 flowing through the power transistor 4, a reference voltage source 12 for generating a reference voltage V.sub.R for comparison with the primary voltage V.sub.D as sensed by the current sensing resistor 11, and a differential amplifier 20 for absorbing a sink current Is from a base current I.sub.B4 supplied to the base of the power transistor 4 in proportion to a deviation or difference of the sensed primary voltage V.sub.D from the reference voltage V.sub.R. The differential amplifier 20 has a first or non-inverted input terminal connected to the reference voltage source 12 so as to be supplied with the reference voltage V.sub.R, a second or inverted input terminal connected to a node between the emitter of the power transistor 4 and the resistor 11 so as to be imposed upon by the primary voltage V.sub.D across the resistor 11, and an output terminal S connected to a node between the base of the power transistor 4 and the junction between the resistor 5 and the collector of the drive transistor 6. The differential amplifier 20 is driven by the sum of an emitter-base voltage of the power transistor 4 and the voltage across the current sensing resistor 11 so as to absorb a part of the base current I.sub.B4 flowing from the storage battery 1 to the base of the power transistor 4 through the resistor 5 as a sink current Is.
FIG. 4 is a circuit diagram showing a more concrete structure of the differential amplifier 20 of FIG. 3. In this figure, the reference voltage source 12 of FIG. 3 comprises a constant current source 12a connected to the storage battery 1 through the resistor 5 for generating a constant current, and an NPN transistor 12b connected between the constant current source 12a and ground. The transistor 12b has a collector connected to the constant current source 12a, a base directly coupled to the collector thereof to form a diode connection, and an emitter grounded. A junction between the constant current source 12a and the collector of the transistor 12b is connected to the first or non-inverted input terminal of the differential amplifier 20 for applying thereto a reference voltage V.sub.R across the transistor 12b.
The differential amplifier 20 includes an NPN transistor 21 which has a base coupled to the junction between the constant current source 12a and the collector of the transistor 12b, the base acting as the first or reference input terminal of the differential amplifier 20, an NPN transistor 22 which has a base connected to a junction between the collector of the power transistor 4 and the resistor 11, the base acting as a second or sensing input terminal of the differential amplifier 20, a PNP transistor 23 having a collector coupled to the collector of the transistor 21, a PNP transistor 24 having a collector coupled to the collector of the transistor 22, an NPN transistor 25 which has a collector coupled to the base of the power transistor 4, the collector acting as the output terminal of the differential amplifier 20, a base coupled to a junction between the collectors of the transistors 21, 23, and an emitter grounded, and a resistor 26 having one end connected to a junction A between the bases of the transistors 21, 22 and the other end connected to ground. The transistors 23, 24 have their emitters coupled together to the base of the power transistor 4 and their bases coupled to each other to form a current mirror circuit. The base and the collector of the transistor 24 are directly connected to each other to form a short circuit.
The operation of the above-mentioned current limiter of FIG. 4 will now be described in detail while referring to FIG. 3. When the drive transistor 6 in the unillustrated ECU is turned off to start the power supply to the ignition coil 2, the source voltage V.sub.B of the storage battery 1 is imposed on the base of the power transistor 4 through the resistor 5, thus turning the transistor 4 on. As a result, a primary current I.sub.1 begins to flow from the primary winding 2a of the ignition coil 2 to the emitter of the power transistor 4 via the collector thereof. A part of the primary current I.sub.1 branches into the current sensing resistor 11 of a limited resistance so that there develops a voltage drop V.sub.D across the resistor 11.
At the same time, the current limiter 10 starts to control the base current I.sub.B4 to the power transistor 4 so that the sensed voltage V.sub.D across the resistor 11 corresponding to the primary current I.sub.1 is made equal to the reference voltage V.sub.R across the collector-emitter of the transistor 12b, as imposed on the base of the transistor 21. That is, when the sensed voltage V.sub.D becomes equal to the reference voltage V.sub.R, a part of base current I.sub.B4, which is to be supplied to the base of the power transistor 11, is absorbed as a so-called sink current Is by the differential amplifier 20, which forms a negative feedback loop, thus reducing the magnitude of the base current I.sub.B4. As a result, the primary current I.sub.1 is controlled or limited to a level corresponding to the predetermined reference voltage V.sub.R. In this connection, as shown in FIG. 4, a constant current is supplied from the constant current source 12a to the transistor 12b of the reference voltage source 12, so the reference voltage V.sub.R imposed on the base of the transistor 21 is held at a constant level. The reference voltage V.sub.R is set in advance to a value equal to the sensed voltage V.sub.D across the resistor 11 generated at the time when the primary current I.sub.1 flowing in the primary winding 2a of the ignition coil 2 reaches a predetermined limit value.
If the sensed voltage V.sub.D exceeds the reference voltage V.sub.R, the base voltage of the transistor 22 at the current-sensing input terminal of the differential amplifier 20 rises while the base voltage of the transistor 21 at the reference input terminal of the differential amplifier 20 remains constant. Consequently, the transistor 22 is being turned on, so the voltage at the junction A between the bases of the transistors 21, 22 is accordingly increasing, causing the transistor 21 in a direction to be turned off.
On the other hand, the transistors 23, 24, which together constitute a current mirror circuit, function to flow currents of the same magnitude, so that as the transistor 21 is being turned off, the current flowing from the transistor 23 into the output transistor 25 is increasing. As a result, the output transistor 25 absorbs a sink current Is from the base current I.sub.B4 to be supplied to the base of the power transistor 4 in proportion to the magnitude of the sensed voltage V.sub.D, whereby the primary current I.sub.1 is accordingly decreased to a predetermined value corresponding to the reference voltage V.sub.R.
In this regard, the current limiter 10 is required to operate such that the differential amplifier 20 functions to control the base current I.sub.B4 of the power transistor 4 on the basis of the sum of the voltage across the base-emitter of the power transistor 4 and the voltage across the current sensing resistor 11. However, the magnitude of the base current I.sub.B4 is so great that if the capacity of the differential amplifier 20 is insufficient, the level of the sensed primary voltage V.sub.D will be offset from the actual level thereof, thus making it difficult for the current limiter 10 to exhibit an expected predetermined current limiting characteristic. That is, even if the sensed voltage V.sub.D exceeds a dynamic input range of the differential amplifier 20 and maximizes the base current to the output transistor 25, the output transistor 25 becomes unable to absorb the base current I.sub.B4 supplied to the base of the power transistor 4 to a satisfactory or sufficient extent. Specifically, if there is an offset in the level of the voltage V.sub.D across the resistor 11 as sensed by the differential amplifier 20, the operating characteristic of the current limiter 10 exhibits voltage dependency. In other words, the operating characteristic varies depending upon the voltage as sensed, so the current limiting value, to which the current limiter 10 limits the primary current I.sub.1, increases such that a large secondary current in excess of a predetermined allowable limit can be developed when the power transistor 4 is turned off.
With the above-described known current limiter for an internal combustion engine in which the sink current Is is absorbed by the single output transistor 25 alone, however, it is difficult to reduce the base current I.sub.B4 to the power transistor 4 to a satisfactory extent, resulting in the problem that an excessive increase in the ignition current flowing in the ignition coil 2 cannot be suppressed.
In order to cope with this situation, it is considered to increase the amplification factor or gain of the output transistor 25, but such a measure is subject to certain limitations and drawbacks. Namely, it is generally known that if the amplification factor is increased above a certain value (i.e., greater than "1") and if there is a phase shift in a feedback signal greater than 180 degrees, the current limiter generally causes oscillations. To prevent this, costly measures must be taken, resulting in a complicated arrangement and an increased manufacturing cost.